RBY has the famous Missingno trick (speak to the guy in Viridian who teaches you how to catch a pokemon, then fly to Cinnabar and surf up and down along the right side of the island). You get an infinite ammount of the 6th item in your bag.
I know a few in Mario Sunshine. Like when you first get 70 shines and Delfina Plaza is flooded. Go find one of the logs floating on the water. In you go under it and jump right, you should fall right through the floor.
Then there's the one where you go into the water behind the shine tower. If you go to the pool closest to the main area (furthest from the entrance to corona mountain). Long jump from the platform above it into the water and hit the wall closest to the tower. You shoudl bounce back and be walking under the water. Jumping brings you back. Alternatevly, if you long jump into the same wall while underwater and ground pound in the middle of the falling back sequence, you can fall through the floor. This will result in either you landing in one of the underground pipe systems or in you landing on the bottom of the level. If you stand on the bottom of the level for more than 2 seconds, you die. but if you keep jumping then you'll be fine.
Also, when riding yoshi, you can lick up a fruit. Jump off of him while his toung is still out. The fruit will still be floating in mid-air. Then the next time you use his tounge, the fuit will teleport onto Yoshi's tounge. This can be useful when you're using him in a battle and theres no fruit around.
Also in Pokemon GS (not crystal) you can duplicate a pokemon by putting them in your box. The game will make you save and you must turn the power off when the game tells you to not turn the power off. When you turn is back on, you will have one copy of the pokemon in your party and a clone of it in the box. Make it hold a rare candy or a masterball for an added effect. There's also a duplication glitch in Emerald but I personally don't know how to do it. But I know it involves the battle frontier.
I know a few in Mario Sunshine. Like when you first get 70 shines and Delfina Plaza is flooded. Go find one of the logs floating on the water. In you go under it and jump right, you should fall right through the floor.
Then there's the one where you go into the water behind the shine tower. If you go to the pool closest to the main area (furthest from the entrance to corona mountain). Long jump from the platform above it into the water and hit the wall closest to the tower. You shoudl bounce back and be walking under the water. Jumping brings you back. Alternatevly, if you long jump into the same wall while underwater and ground pound in the middle of the falling back sequence, you can fall through the floor. This will result in either you landing in one of the underground pipe systems or in you landing on the bottom of the level. If you stand on the bottom of the level for more than 2 seconds, you die. but if you keep jumping then you'll be fine.
Also, when riding yoshi, you can lick up a fruit. Jump off of him while his toung is still out. The fruit will still be floating in mid-air. Then the next time you use his tounge, the fuit will teleport onto Yoshi's tounge. This can be useful when you're using him in a battle and theres no fruit around.
Also in Pokemon GS (not crystal) you can duplicate a pokemon by putting them in your box. The game will make you save and you must turn the power off when the game tells you to not turn the power off. When you turn is back on, you will have one copy of the pokemon in your party and a clone of it in the box. Make it hold a rare candy or a masterball for an added effect. There's also a duplication glitch in Emerald but I personally don't know how to do it. But I know it involves the battle frontier.